This invention relates to the art of stall indicators or warning devices for use in fixed-wing aircraft. A sample of the multitude of stall warning devices or systems to be found among the US Patents is listed in the Information Disclosure Statement.
Unexpected wing stall during aircraft flight operations is a serious phenomenon that has led to a significant proportion of small aircraft accidents and deaths. At busy or stressful times, a pilot may approach this condition without awareness, even though an alert pilot can notice certain signs of impending stall. Typically, this condition occurs at low altitude while banking for a turn in the “pattern” prior to landing, especially under gusty wind conditions that can result in sudden changes of airspeed. Lift diminishes and drag increases, snowballing the condition. Stall can also happen when pulling out of a dive or other condition of increased acceleration (Gs) due to excessive pitch control action. Generally, the nose, and frequently one wing, of the aircraft drops, more or less suddenly, and it is possible that the aircraft may go into a spin. The pilot must react quickly (at least to ease the stick forward to reduce angle of attack), but he may not if surprised.
Specifically, stall of a fixed-wing aircraft is the condition where the angle of onrushing airflow relative to the wing chord (the “angle of attack”, i.e., AoA) exceeds the angle of maximum lift, causing separation of the airflow from the wing surface and loss of lift. In particular, it happens when attempting to fly the aircraft below the airspeed (the “stall speed”) that develops just enough lift to support the effective weight (under G-loading) of the aircraft at the angle of attack of maximum lift.
It is common to provide some kind of automatic stall sensor and alarm system to bring an impending stall to the instant attention of the pilot so that s/he will react with appropriate evasive action. However, there are some production and experimental aircraft that do not have stall warning devices. Most gliders don't, one reason being that they usually have removable wings that make placement and reconnection of such devices inconvenient or unreliable, or subject them to damage. Experimental and ultralight aircraft frequently don't have stall sensors because of cost, uncertainty of calibration, or the sense that “they're not needed”.
Numerous devices or combinations of sensors, computers and attention-raisers have been employed to warn of such an impending or occurring event so the pilot can take control action to avert or correct the outcome. The most direct approach to sensing stall is to measure the angle of attack that the wing experiences, so impending stall is often sensed by monitoring critical changes in airflow or pressure distribution relative to the wing leading edge, as done by the first four patent references.
One type frequently seen on smaller general aviation is based upon a sensor located upon a wing in a position that can directly detect changes in airflow dependent upon changes in the angle of attack. Specifically, near the leading edge of the wing, a precisely located open slot leading to a resonance chamber will emit a loud warning sound when stall is impending, or a vane type facing into the airflow will flip a switch when the critical angle of stall is approached (Blair and Andrew), or pressure or flow sensors at two spaced locations on the leading edge are differentially compared to obtain a signal dependent upon angle of attack (Maris and Corey).
Several other basic types of devices or systems can accomplish the stated purpose. The Dendy and Quinlivan patents are typical of systems that employ a multiplicity of sensors that may be located at various control locations on an aircraft, their outputs all combined in a computer to yield a signal on an appropriate indicator to command the pilot's attention or even take automatic corrective measures. This type tends to be seen on large commercial or military aircraft.
The remaining five cited patents are all based upon a different principle from those that directly are mounted near or on a wing to determine angle of attack from the airflow. Those five may share some of the types of sensing and computation involved in Dendy and Quinliven, but are mostly located in a single device on the instrument panel of the aircraft and primarily have connections to the usual static and pitot (dynamic) pressure sources, plus a battery or other source of electricity, and they all sense normal accelerations, parallel to the lift vector. Specifically, these five patents all operate on the principle of balancing the forces of normal accelerations due to gravity and maneuvering (i.e., G-forces) against a force proportional to dynamic pressure (which in turn is proportional to the square of airspeed), and therefrom developing a measure of angle of attack.
The current invention is similar in its underlying use of the same basic principle as the last five cited, but an optional new physical implementation and important safety features are added in the instant device to enhance that function and to extend warnings to other unusual and dangerous aerodynamic events.